One of the Grand Challenges in science and engineering is the demonstration of inertial confinement fusion (ICF)—thermonuclear ignition and net energy gain—in the laboratory. Lawrence Livermore National Laboratory (LLNL), in collaboration with its partners in the National Ignition Campaign (NIC)—Los Alamos and Sandia National Laboratories, the Laboratory for Laser Energetics at the University of Rochester and General Atomics of San Diego are tackling this challenge with a credible attempt at ignition on the National Ignition Facility (NIF). NIC is the “bridge” that will take NIF to routine operations as a highly flexible high energy density science facility by 2013.
The experiments at the NIF use a target that is precisely located in the center of the NIF target chamber. Just a few millimeters long, NIF targets are complicated, precision assemblies, often requiring novel material structures. Creating these targets is a complex interplay among target designers, materials scientists and precision engineers. The NIC team is perfecting ICF experiment target fabrication and materials, as well as advancing laser driver performance, target design, and the performance of experimental diagnostics.
Manufacturing requirements for the NIF targets are extremely rigid, including components that are machined to an accuracy of within one micron. Precise microassembly of the targets involves assembly with micron-level accuracy. The extreme temperatures and pressures the targets encounter during experiments make the results highly susceptible to any imperfections in fabrication.
To achieve ignition, the target design contains four components: deuterium and tritium (D-T) fuel, a capsule with fill tube, a hohlraum, and thermal control hardware. A hohlraum is the metal case that surrounds a fuel capsule for NIF experiments. NIF's powerful laser beams impinge on the inside of a hohlraum, where the laser energy is converted to X-ray energy. These X-rays bathe the fuel capsule and rapidly ablate, or burn away, the capsule's outer layer. The principle of conservation of momentum (every action requires an equal and opposite reaction) forces the remaining material to implode or compress. Compression of the D-T fuel—which has been formed in an ice layer inside the capsule—to extraordinarily high temperature, pressure and density ignites a burning hydrogen plasma. To assure the symmetry of the implosion, the capsule is placed within a number of microns of the center of the hohlraum, which is itself just one centimeter across.
Once assembly is complete, the target is integrated with the cryogenic target positioning system (CTS) in NIF. This system forms and characterizes the D-T layer and places the target at target chamber center for the shot. Target position is maintained to within several microns, and temperature is typically held in the range of 18 to 20 Kelvin (−427 to −424 degrees Fahrenheit) with a stability of about one thousandth of a Kelvin.
Current objectives for the NIC include the fabrication of one target per day while maintaining the flexibility to accommodate changes in target parameters resulting from knowledge gained during the campaign. Historically, building ICF targets depended on a significant amount of hand-craft and skill. Using that method, building an ignition target typically required a one-week effort by a team of two to three people and resulted in inconsistent quality.
Therefore, there is a need in the art for improved methods and systems related to assembly of fusion targets and other miniaturized devices.